1. Field of the Invention
The present invention relates generally to a borehole strainmeter or extensometer, and more specifically, to a recoverable, multiple position strainmeter for a borehole for measuring changes in strain along the borehole. Particular utility for the present invention is found in the area of measurement of changes of loading strain in support piles, although other utilities are also contemplated.
2. Brief Description of Related Prior Art
Piles are load-bearing supports used to construct foundations of buildings and other large structures. Support piles are normally of two types, driven or cast-in-place. Driven piles are usually made from steel beams or steel pipes that are filled with concrete after driving and precast concrete segments. Cast-in-place piles are usually made from concrete which is cast-in-place in large diameter drilled holes in soil and/or rock or from a combination of steel beams and concrete cast-in-place in drilled holes.
Borehole strainmeters are instruments that are used to measure changes in strain in piles after the pile is in position. The strainmeter measures relative displacements of the pile at various depths in the pile. Such measurements permit the load bearing capacity and load distribution characteristics of the pile and surrounding ground to be determined. This permits the relative stability of the pile and soil surrounding the borehole to be determined, so as to be able to decide whether they are adequate for the intended structure to be constructed.
Borehole strainmeters typically comprise three main components: at least one displacement sensor, one or more anchors at various depths in the pile, and rods or wires for interconnecting the anchors with the displacement sensor. The sensor is usually mounted inside a reference head located in the pile and includes an electrical or mechanical readout. Changes in strain distribution along the pile due to changes in loading of the pile and/or shifting of the distribution of the surrounding soil mass are reflected in changes of relative axial displacements between the anchors, which are measured by the displacement sensor.
As used herein, a borehole strainmeter with a single anchor and a rod or wire extending between the anchor and the reference head is defined as a "single position borehole strainmeter." A strainmeter with a plurality of anchors is defined as a "multiple position borehole strainmeter."
Most conventional borehole strainmeters are designed to be permanently embedded within the pile (with e.g., concrete), and are cast or driven together with the pile into the borehole. Disadvantageously, such strainmeters cannot be recovered (i.e., removed from the pile) after use (e.g., after sufficient strain measurement data are collected). This is economically wasteful. Also disadvantageously, given the harshness of the installation process, quite often such strainmeters are damaged or destroyed while being cast or driven with the pile into the borehole.
One type of recoverable prior art borehole extensometer is disclosed in U.S. Pat. No. 4,719,803 to Capelle et al. The extensometer disclosed in Capelle et al. is positionable in a borehole for monitoring relative mass displacements axially along the borehole between at least two spaced-apart anchor points. The strainmeter includes at least two anchor members arranged in spaced-apart opposite relationship to define the at least two anchor points. Each of the anchor members have radially displaceable borehole wall contacting means and include mechanical means for radially outwardly displacing the borehole wall contacting means for engaging the borehole wall and to thereby fix the anchor member in the borehole against the wall thereof. A linear displacement sensing means is arranged between the anchor members, connected to one of the anchor members, and includes an elongated axially movable sensor element. The sensor element is normally biased in a direction toward the other anchor member for contact engagement therewith. Relative mass displacements are transmitted to the sensor element and are measured by the displacement sensing means as a variation in distance between the anchor members within the borehole. The anchor members each have a cylindrical body and multiple elongated axially extending borehole wall contacting elements arranged circumferentially of the body. The mechanical means includes axially movable cams arranged in the body and cam follower means connected to the radially displaceable borehole wall contacting element and cooperable with the cam means upon movement thereof for radially outwardly displacing the element to contact the borehole. Connection rods are preferably used to connect the anchors and sensor.
According to Capelle et al., their extensometer must be assembled, element by element, inside the borehole, and the anchors must be separately mechanically actuated from within the borehole, using specialized tools including setting tubes and rods inserted into the borehole. In practice, these are cumbersome and time-consuming processes, given the fact that most boreholes are no more than several inches in diameter, and it is difficult to assemble and actuate the extensometer in such a confined space. Indeed, it has been found that installation of the Capelle et al. device into boreholes of one hundred feet or more in depth may take several hours. Additionally, in the Capelle et al. extensometer, the displacement transducer is compressively spring-loaded against the anchors. This compressive biasing can cause the connecting rod between the anchors to bend or buckle, thereby decreasing the extensometer's measurement accuracy, especially if the rod is greater than several several feet in length. Finally, in order to remove the Capelle et al. extensometer from the borehole, the cumbersome and time-consuming processes used to assemble the extensometer and actuate the anchors must be reversed to dis-assemble and deactuate the extensometer and anchors.
Other prior art devices of general relevance to the subject invention are disclosed in U.S. Pat. No. 4,844,197 to Maissa and U.S. Pat. No. 5,060,751 to Kuhlman et al. These devices suffer from the aforesaid and/or other disadvantages and drawbacks.